As top plates used for electromagnetic cooking appliances and infrared cooking appliances, low thermal expansion glass, ceramics or like material is used. Low thermal expansion glass, ceramics or like material has also come into use for top plates for gas cooking appliances because of its excellent aesthetic quality and cleanability. In these top plates using such glass or like material, a metal film, such as a Ti film, is formed on a substrate in order to visually hide internal components, such as a cooking appliance and a temperature sensor. The metal film gives a beauty of metallic luster or the like to the top plate.
FIG. 2 is a cross-sectional view showing such a conventional top plate for a cooking appliance. As shown in FIG. 2, a top plate 10 for a cooking appliance is provided with a light shielding layer 2 made of a metal film on a glass substrate 1, and a protective layer 3 made of a SiN film or the like on the light shielding layer 2. The light shielding layer 2 is provided, as described above, in order to give metallic luster to visually hide the internal cooking appliance. The protective layer 3 is provided in order to prevent the light shielding layer from being oxidized by heat application.
In the conventional top plate, a Ti film, for example, is formed as a light shielding layer 2 as described above. The use of a Ti film as a light shielding layer 2 provides good heat resistance. Meanwhile, the Ti film is a metal film having a high electrical conductivity. If a touch sensor part 4 is formed as shown in FIG. 2, an insulating film 7 must be provided in a region of the Ti film corresponding to the touch sensor part 4. A conductive ink layer 5 is formed in the touch sensor part 4 on the top surface of the substrate 1. By forming the insulating film 7, an electrostatic capacitance can be created between the conductive ink layer 5 and an electrode 6 disposed on the opposite side of the insulating film 7 to the conductive ink layer 5, thereby forming a capacitance switch.
Thus, in such a conventional top plate using as a light shielding layer 2 a good-conductivity metal film, such as a Ti film, an insulating film must be formed in a region of the top plate in which a touch sensor part is formed, in place of the light shielding layer and the protective layer. Therefore, there exists the need to form an insulating film, for example, by printing, mask the part in which the insulating film has been formed, and then form a light shielding layer and a protective layer. This poses the problem of complicated production process. Furthermore, the part in which the insulating film has been formed is different in color from the other part. Therefore, the problem of impaired aesthetic quality also arises. Hence, there is a demand for a top plate for a cooking appliance which can be provided with a touch sensor part without involving the provision of an insulating film.
Among recent known cooking appliances are those in which a light source, such as an LED, is internally disposed to use light from the light source as an indication that indicates the intensity of heating power or the like. However, conventional top plates for cooking appliances have low transmittance of light at short wavelengths and therefore have the problem of difficulty in transmitting such LED light, particularly blue LED light.
Patent Document 1 proposes a top plate for a cooking appliance in which a metal film made of one of various metals including Si and Ti is used as a light shielding layer. However, this conventional technique is intended to provide a top plate for a cooking appliance which can be given various colors and has excellent heat resistance, and is not contemplated to enhance the visibility of light for indication from a blue LED or the like. In addition, according to this conventional technique, the top plate could be given high marks for heat resistance but cannot satisfy a stricter standard of heat resistance. Patent Document 1: Published Japanese Patent Application No. 2004-333102